This invention relates to drivers, for example, an ECL source terminated transmission line driver.
An ECL transmission line driver charges up a voltage on a transmission line in response to a logic high voltage level appearing at an output of an ECL gate. Further, in response to logic low voltage level appearing at the output of the ECL gate, the ECL transmission line driver discharges the voltage on the transmission line to a logic low voltage level. It is well known that ECL drivers typically have good high frequency performance. However, ECL drivers dissipate high power in a steady state condition as well as when switching from one logic state to another.
One example of an ECL transmission line driver includes a first transistor having a base coupled to the output of an ECL gate. The emitter of the first transistor is coupled to a transmission line through a source terminated resistor. A second transistor, having a base coupled to a reference voltage and a collector coupled to the emitter of the first transistor, sinks a predetermined current at its collector. Briefly, when the output of the ECL gate switches to a logic high state, the first transistor is rendered operative thereby causing a positive voltage transition at the emitter of the first transistor and on the transmission line. Further, the predetermined current sunk by the second transistor is sourced from the emitter of the first transistor. On the other hand, when the output of the ECL gate switches to a logic low state, the first transistor is rendered non-operative while the second transistor sinks the predetermined current from the transmission line to cause a negative voltage transition on the transmission line. Thus, it must be realized that the second transistor always sinks the predetermined current regardless of the voltage level appearing at the output of the ECL gate. As a result, the above described source terminated transmission line driver is not power efficient since the second transistor must always sink a predetermined current wherein this predetermined current must be large enough to cause a negative voltage transition on the transmission line even when the output of the ECL gate is at a logic high voltage level.
Hence, a need exists for a source terminated transmission line driver circuit that adequately charges and discharges a transmission line to respective high and low voltage levels while minimizing power dissipation.